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Normal Faults are inclined
dip-slip
faults
that
accommodate the extension of
the Earth’s crust. Along a
normal fault, the hanging wall
block
moves
downward
relative to the footwall block.
Normal
faults
generally
emplace younger rocks on top
of older layers, such that
stratigraphic section is missing
in a vertical section through
the fault.
4.1 Characteristics of Normal Faulting
The separations caused by normal faulting
parallel to the strike and the dip depend
on the relative orientations of the fault and
the stratigraphic layering.
In areas where flat-lying beds are
deformed by normal faults, rollover folds
in the hanging wall block are common.
The beds in the hanging wall block tilt
down into the fault.

Listric normal faults: concave
upward faults whose dip
decreases with increasing depth.
Features of fault surfaces are variable and
depend on the fault’s shape, the depth of
movement, and brittle vs. ductile
deformation. Surface features such as
cataclastic rocks and slickensides are
evidence of normal faulting in the field.
4.2 Shape and Displacement of Normal
Faults
The surface trace of a normal fault is not
generally a straight line, but rather may be
a curve or series of connected line
segments.
Because normal faults do not always
maintain a constant dip with increasing
depth, a cross section fails to capture a
fault’s complete geometry.

Detachment fault: a low-angle
fault that marks a major
boundary between unfaulted
rocks below and a hanging wall
block above that is commonly
deformed and faulted.

Imbricate faults: closely spaced
parallel faults of the same type
that either terminate against the
detachment fault or merge with
it.
Displacement on ideal normal faults is
parallel to the dip of the fault surface, but
the hanging wall’s rigid movement
relative to the footwall block is not down
the dip along an entire fault. Movement
on normal faults can be nonrotational or
rotational depending on whether the fault
blocks’ orientations remain constant
through the faulting. Thus, a hanging wall
block moving over a fault must deform
internally
4.3 Structural Associations of Normal
Faults
Normal faults are generally present as
systems of many associated faults (see
figure 4.4, pg. 94)

Synthetic faults: small scale
faults parallel to the major fault
with the same sense of shear.

Antithetic faults: small scale
faults with comparable dips but
in the opposite dip direction from
the main faults.

Graben: a down dropped block
bounded on both sides by
conjugate normal faults that dip
toward the down-dropped block
on both sides.

Half-graben: a down-dropped
tilted block bounded on only one
side by a major normal fault.

Horst: an uplifted block
bounded by two conjugate
normal faults that dip away from
the uplifted block on both sides.

Horst-and-graben
structure:
alternating uplifted and downdropped fault blocks.
Faults and sediments can reveal when
major periods of uplift occurred as well as
the sequence and the time of exposure of
the different rock types in the uplifted
fault blocks, known as the unroofing
sequence.
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Local normal fault systems are frequently
associated with other structures that
require extension of crustal layers, such as
domes, folds, cavities, and pull-apart
structures on strike-slip faults.

Ring faults: a system of
concentric normal faults into
which surficial rocks collapse
when a cavity forms at depth.

Diatremes:
volcanic
pipes
explosively blasted through
crustal rocks
Regional systems of normal faults exist
all around the world (e.g. Basin and
Range province in N. America) in which
the faults form in conjugate sets and have
roughly the same strike but have dips of
varying
magnitude
and
opposite
directions.

Transfer zone: area between
normal
faults
in
which
deformation is accommodated by
folding, faulting, and fracturing.

Transfer faults: transfer zones
that are distinct strike-slip faults.
In extreme extensional areas, normal
faulting strips off the shallow layers of
rock to expose originally deeper crustal
rocks.

Metamorphic core complex:
exposures of deep crust exhumed
in association with largely
amagmatic extension.
A detachment fault often has a corrugated
shape, with the axis of the corrugations
parallel to the slip direction of the fault.
The corrugations are an original structure
of the fault (mega-mullion structure).
The footwall basement rocks and the
detachment fault may form a structural
dome (turtle-back).
Buried normal fault systems along rifted
continental margins, e.g. Gulf Coast,
resemble exposed rifted regions. The Gulf
Coast region is characterized by thick
sediment, subsidence, and normal fault
systems. Many normal faults here are
growth
faults,
or
regional
contemporaneous faults, that are active
during sedimentation and form by
differential compaction of shale layers or
by gravity sliding toward the basin. These
faults are characterized by stratigraphic
Daniel Jensen, 2011
Edited by Curtis Baden and Bridget Floyd, 2013
sequences with units thicker on the
hanging wall block than they are on the
footwall block. In the Gulf Coast region,
the underlying salt formations have
interacted with these fault systems to form
large-scale hydrocarbon traps of great
interest to the petroleum industry.
4.4 Kinematic Models of Normal Fault
Systems
A kinematic model of a fault system is a
description of the motions that have
occurred on the faults in it. Kinematic
models depicted in cross section are
typically oversimplified in their inferred
geometry and tectonic behavior. Fault
termination lines can easily be left out due
to lack of data, and many models fail to
conserve mass within the faulted system.
See figures 4.19-4.22 (pg. 108) for listric
normal fault kinematic models. Note that
true fault systems are generally far more
complex than our cross-sectional models
can depict.
applicability of the model (see pg. 112 for
equations), but it can be used for
approximation of extension along major
faults.
Palinspastic restoration can be used to
construct a balanced cross section parallel
to the slip direction and restore the
geology to its original configuration prior
to deformation. This technique requires
excellent data from both the subsurface
(geophysical methods) and the surface,
however,
severely
limiting
the
applicability of this approach. Eroded
fault systems can introduce difficulty in
that the precise original geometry and
curvature of the fault can be lost.
References & Resources
Robert J. Twiss, Eldridge M. Moores,
Structural Geology 2nd edition, (W. H.
Freeman), p. 91-114, 2006

Extensional Duplex: a stack of
horses that are progressively cut
from the footwall block and
transferred to the hanging wall
block.
Floor Fault: The bottom of the duplex. It
is also the active fault, while the roof fault
bounding the top of the duplex is never
active as a single fault.
Figure 4.23 depicts two models of
balanced, complete cross sections of
normal fault systems. Both models
account for all fault termination lines and
for all required tectonic motions (lacking
in simplified cross sections).
4.5
Determination
of
Extension
Associated with Normal Faults
Estimates of extension based on fault
geometry
Extension can be defined as the change in
length in a given direction caused by
deformation of the system, devided by the
original length. To quantitatively evaluate
the extension across a region using fault
geometry, assumptions regarding fault
strike uniformity and a region’s change in
length must be made. This model assumes
that extension is accommodated by the
largest faults within a system, and
assumes extension along the smaller faults
to be negligible. Assumptions limit the
Daniel Jensen, 2011
Edited by Curtis Baden and Bridget Floyd, 2013
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